vere: report, not crash, on unreasonable -Y and -Z
vere: report, not crash, on unreasonable -Y and -Z

Previously, we would crash on attempting to +scot cells, or parse
invalid paths. Now we catch those cases, and print legible error
messages instead.

ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit